1. Field of the Invention
The present invention relates to a transmitter which amplifies phase-modulated signals with a high-frequency power amplifier, and transmits them, and, more particularly, to a transmitter in which AM/PM conversion distortion in the transmission power of a mobile satellite communication terminal is reduced.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the configuration of a conventional transmitter. In the figure, input data for the transmitter is two series of transmission data signals D.sub.I and D.sub.Q consisting of an in-phase channel (I channel) and a quadrature channel (Q channel) which are orthogonal binary baseband signals. This data is called an NRZ signal.
In addition, such a transmitter comprises a Nyquist filter 11 for eliminating harmonic components in the baseband, a four-phase modulator 13 receiving the output from the Nyquist filter 11 as its input and outputting four-phase modulated signals, and a power amplifier 14 for frequency-converting the modulated signals (QPSK signals) from the four-phase modulator to high-frequency signals in the microwave band, and transmitting high-power signals in the microwave band. Such configuration is described in, for example, Japanese Published Unexamined Patent Application No. 3-171953, by Minori Kouno.
Generally, QPSK signals not bandwidth-limited have a constant envelope for modulated signals, and, therefore, are not affected by the nonlinearity of the transmission path (for example, AM-AM conversion or AM-PM conversion where AM means amplitude modulation and PM means phase modulation).
However, bandwidth-limited QPSK signals are widened for their spectrum by the nonlinear amplification of a power amplifier and affected by AM-PM conversion and the like so that the code error rate characteristics are deteriorated. Therefore, if it is desired that these not be affected by the nonlinear amplification of the power amplifier and the like, the modulation characteristics of the envelope must be maintained as constant as possible. The above operations are fully described in "TDMA Communication," S. Kato et al., IECE Japan (1989), pages 61-76. The impact of such nonlinearity of the power amplifier is described for the configuration in FIG. 1.
In the transmitter in FIG. 1, the output of the four-phase modulator 13 is usually bandwidth-limited by using a bandwidth filter to prevent unnecessary spurious noise in the radio frequency range and interference with adjacent signals.
The bandwidth-limited modulated signals are input to a high-frequency power amplifier 14. Because the power amplifier 14 used in the transmitter of a mobile satellite communication terminal is particularly required to consume minimum power, it is desirable that it be as much in the nonlinear amplification region of input/output characteristics as possible. If it is to be used in the nonlinear region of input/output characteristics, however, AM-PM conversion distortion is generated along with changes in the amplitude at each phase of the output signal from the four-phase modulator 13, which, in turn, generates phase error in the transmission signals so that phase error in the transmission signals is increased.
The transmission signals with such phase error are transmitted to another earth station via a satellite. In a case where the above-mentioned modulated signals are decoded into two series of data signals of the orthogonal I and Q channels by a decoder at the receiving side, decoding directly causes demodulation errors in the output of an orthogonal detector to cause a degradation in the error rate. For example, when a class B or C high-frequency FET amplifier is used as the power amplifier 14 and operated near the saturation point, the AM-PM conversion characteristics would be as high a value as 8-12 deg/dB.
As a result, the AM-PM conversion distortion generated in the output of the transmitter causes a phase error in the output of the modulator at the receiving side so that the value of Eb/No in the Eb/No to bit error rate characteristics would be lowered by about 0.3 dB when compared with theoretical non distortion. Here, Eb/No is the ratio of the noise power density in the input of the receiver (W/Hz) to the power per bit in the input of the receiver (W).
The lowering of Eb/No causes serious problems such as a need to review the circuit design or change the antenna size as in the case of satellite communication where the operation should be performed at a low Eb/No as possible so as to transmit as much information.